The biologic, functional and chemical characterization of human lymphoid cell surface antigens controlled by genes of the major histocompatibility complex is under investigation. Antigens heretofore thought specific for B cells and monocytes have been identified on T cells, albeit in lesser quantities. With appropriate stimulation, T cells can be induced to become functionally suppressive for the production of immunoglobulins. These cells bear abundant B cell antigens. A heteroantisera which specifically reacts with suppressor cells has been developed. Serologic and immunochemical techniques have demonstrated that 2 gene loci control the expression of B cell alloantigens. B cell alloantigens associated with diseases, Sicca syndrome, SLE, and psoriasis have been identified, the results suggesting an autoimmune response gene. A diabetic population demonstrating allergic reactions to insulin was studied. A specific B cell alloantigen appears to mark those individuals demonstrating allergic manifestations to pork insulin. Amino acid sequence analysis of HLA alloantigens controlled by two genes in the major histocompatibility complex is near completion. Significant homologies in these two gene products is demonstrated.